Electric discharge device having improved dispenser cathode



Sept. 27, 1966 J. M. HOUSTON 3,275,873,

ELECTRIC DISCHARGE DEVICE HAVING IMPROVED DISPENSER CATHODE Filed July 1, 1964 /n venfor: John M. Hausfon,

H/s Affom e y.

United States Patent 3,275,873 ELECTRIC DISCHARGE DEVICE HAVING IMPROVED DISPENSER CATHODE John M. Houston, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed July 1, 1964. 'Ser. No. 379,659 2 Claims. (Cl. 313-182) The present invention rel-ates to improved electric discharge devices of the high vacuum type and particularly such devices having an improved dispenser type cathode.

In a known type of electron emitting cathode for high power electron tubes a tungsten matrix impregnated with thorium metal is employed. By operating such a cathode at a temperature of about 2200 K., for example, one can obtain a high electron emission density in the order of amperes per square centimeter. However, the life of such cathodes is fairly short since the thorium tends to evaporate and is not automatically replenished. In accordance with an important aspect of the present invention an electric discharge device provided with a refractory metal cathode base is continually supplied with thorium during operation of the device from thorium iodide vapor contained within the device and at a pressure less than 10 mm. of mercury. Such a pressure preserves the high vacuum characteristics of the device and makes possible the operation of such devices as magnetrons and ordinary space charge high vacuum tubes, particularly at high frequencies. Since these devices tend to become smaller as the operating frequency is increased, it is particularly important to provide high emission densities, since the available area of the cathode in many tube configurations is limited as the operating frequency increases. determined by the temperature of the coolest surface exposed to the interior of the tube and in order to dissociate the thorium iodide at the cathode surfacethe cathode should be the hottest surface exposed to the interior of the envelope. These relative temperatures may be maintained from the geometry of the tube and the normal heat input such as the heating energy supplied to the cathode and also the dissipation of the electrodes due to the discharge current. As :an alternative, or additionally, the temperature of the entire device may be controlled by insertion in an oven, as will be described in more detail at a later point.

Accordingly, an important object of my invention is to provide a high vacuum electric discharge device having a dispenser type cathode which provides high emission densities, has long life and is ruggedgbeing automatically self-replenishing and self-repairing after over-heating and the like.

Further objects and advantages of my invention will become apparent as the following descritpion proceesds, reference being had to the accompanying drawing, in which:

FIG. 1 is a schematic representation of one embodiment of my invention; and

FIG. 2 is a schematic representation of a modified form of my invention.

While my invention may be applied to many different types of electric discharge devices, for simplicity of illustration and description a simple diode has been illustrated in the drawing. In the construction shown in FIG. 1, the discharge device includes a vacuum tight envelope 10 of glass or ceramic having a planar anode 11 supported from a lead-in conductor 12. A filamentary cathode of refractory metal 13 is supported from the opposite end of the envelope by a lead-in conductors 14 and 15 sealed to the envelope wall. While the cathode may be of any suitable refractory metal, such as tungsten, rhenium, tanta- The vapor pressure of the thorium iodide is lum or molybdenum, these materials being operable in the range of 2000 to 2400" K., I prefer to use tungsten as the base cathode metal. In the particular arrangement shown, the cathode is adapted to be brought to a temperature in the above range by means of a heating current supplied to the lead-in conductors 14 and 15.

As a means of supplying thorium iodide vapor to the interior of the envelope and at the same time providing for the maintenance of the solid thorium iodide at the lowest temperature of any of the parts exposed to the interior of the envelope 10, I provide a well or tubulation 16 communicating with the interior of the device and enclosing at its lower end a quantity of solid thorium iodide 17. A small amount of glass or mineral wool illustrated at 18 may be inserted in the tubulation above the thorium iodide to insure that it does not fall from its desired position in handling or shipment of the device.

While the location and length of the tubulation with respect to the other parts of the tube may be such as to insure that it is maintained at the lowest temperature of any of the parts exposed to the interior of the tube during operation of the device, this same result may also be accomplished by enclosing the entire device within an oven, illustrated schematically at 19, which may be heated in any convenient manner. As a further means of controlling the temperature the end of the tubulation or a metal member connected therewith may extend through the oven wall to further reduce the temperature of the thorium iodide 18. The manner in which a supply of free thorium on the surface of the cathode 13 is maintained by the discharge device described above will become :clear from a description of the operation of this device and the operating conditions that are maintained.

As indicated above, the vapor pressure of the thorium iodide is determined by the temperature of the coolest surface exposed to the interior of the device. In the arrangement shown in FIG. 1, this is the temperature of the solid thorium iodide 17 in the tubulation 16. The vapor. pressures of thorium iodide for different temperatures within the tubulation are as follows: 350 C.10 torr, 274 C.2.5 1O' torr, 255 C.4 1() torr, 233 C.6 10 torr, 200 C.-10- torr. Examples of the emission densities produced under plused anode conditions give an indication of the emission capabilities of this cathode. At a temperature of 2200 K. for the cathode and a thorium iodide pressure of 4 1O- torr, corresponding to a thorium iodide temperature of 255 C., and for anode pulsed conditions, the emission density was 12.5 amperes per square centimeter of cathode area. The pulses were of 10 microseconds duration and occurred at a repetition rate of '60 pulses per second. For the same cathode operating at 2200 K. and a thorium iodide pressure of 2.5 1() torr, corresponding to a solid thorium iodide temperature of 274 C., the emission density was 24.6 amperes per square centimeter of cathode under pulsed conditions. The continuous emission density of the cathode under the conditions specified in the above two examples was much lower than the pulsed emission. This resulted from poisoning of the cathode due to gases liberated by the hot anode. This poisoning could, as will be readily appreciated by those skilled in the art, be minimized or eliminated by more adequate cooling of the anode and by the provision of suitable gettering means within the envelope so that the continuous emission would be of essentially the same magnitude as the pulsed emission.

These levels of emission density, combined with the long life and self-reconstructing capabilities of the cathode, render it very desirable in applications requiring a high emission density from a long life high temperature cathode.

The cycle by which the thorium iodide dissociates to dispense thorium to the cathode and then the iodide vapor recombines to reestablish the thorium iodide pressure will now be described. When a thorium iodide molecule strikes the hot cathode it decomposes, leaving thorium on the surfaceof the cathode and releasing iodine gas. This gas recombines with thorium as it is evaporated from'the cathode to reestablish the thorium iodide and make it available for another cycle of decomposition and recombination. Although not essential, it may be desirable to insure against any buildup of iodine pressure by establishing in the device a thorium surface maintained at some temperature .at which it readily recombines with the iodine vapor. This may be accomplished by vacuum depositing a thin layer of thorium on the surface of the anode. While the temperature of the thorium metal is not very critical, it should be above the temperature of the solid thorium iodide and substantially below the temperature of the cathode. A temperature between 260 C. and 500 C. is suitable.

If it is assumed that every thorium iodide molecule that strikes the cathode is decomposed, then for a cathode operating temperature of 2200 K., as described in the above examples, the thorium deposited on the cathode for subsequent evaporation is suflicient to maintain the cathode in an activated state, ie maintain approximately one monolayer of thorium on the cathode surface.

In FIG. 2 is shown a modification of my invention in which the envelope is substantially metal separated by a ceramic spacer. In this modification, the envelope is formed of two essentially cup-shaped members 20 and 21 having outwardly extending flanges which are bonded to a ceramic insulating spacer 22 by one of the known high-temperature bonding processes. anode 23 in the form of a disk of a metal such as molybdenum is bonded to the interior of the anode cup 20 and an indirectly heated cathode is supported from the cathode cup 21. The cathode may be in the form of a disk 24 of tungsten or other refractory metal supported from an upstanding flange 25 on the cathode cup 21 by a thin metal cylinder of refractory metal 26 such as tantalum. The cathode 24 is supported in opposed and closely spaced relation with respect to the anode 23. The cathode is heated by radiation and/ or electron bombardment from a filamentary resistance-type heater 27 supported from and adapted to be energized by lead-in conductors 28, sealed through the envelope by means of eyelet seals 29 and 30. The thorium iodide vapor is again furnished A refractory metal by a quantity ofthorium iodide 31 received in a tubulation 32 and retained in position by mineral or glass; 7 wool 33. I

It is believed apparent that, with the anode dissipation and the heat supplied to the cathode by the heaterelement 27, the tubulation, particularly if the Wall section is maintained thin and of suitable length, may inherently posed interior of'the envelope.

tion of the oven or even extending through a wall of, the oven if desired.

What I claim as new and desire to secure by Letters Patent of the United States is: V

1. An electric discharge device of the high vacuum type comprising an evacuated envelope, a plurality of elec-,

trodes exposed to the interior of said envelope including a cathode of refractory metal, a quantity, of, solid thorium iodide exposed to the interior of said. envelope, means for maintaining said iodide at a temperature less than the parts of said device within'said envelope and below.

350 C., and means maintaining said cathode at a higher temperature than the other parts: of said device within said envelope so that said iodide is dissociatedat said cathode to dispense thorium to said cathode during operation of thedevice.

2. An electric discharge device of the high vacuum.

type comprising .an evacuated envelope, a plurality of electrodes exposed to the interior of said envelope including a cathode of refractory metal, a quantity of solid thorium iodide exposed to the interior of said envelope,,means for maintaining said iodide at a temperature less than the parts of said device within said envelope and below 350,

0., means maintaining said cathode at a higher temperature than the other parts of said device within said envelope so that said iodide is dissociated at said cathode to dispense thorium to said cathode during operation of the device, and a thorium metal. surface within saidv envelope for preventing an accumulation of iodide vapor;

No references cited.

JOHN W. HUCKERT, Primary Examiner.

A. J. JAMES, Assistant Examiner. 

1. AN ELECTRIC DISCHARGE DEVICE OF THE HIGH VACUUM TYPE COMPRISING AN EVACUATED ENVELOPE, A PLURALITY OF ELECTRODES EXPOSED TO THE INTERIOR OF SAID ENVELOPE INCLUDING A CATHODE OF REFRACTORY METAL, A QUANTITY OF SOLID THORIUM IODIDE EXPOSED TO THE INTERIOR OF SAID ENVELOPE, MEANS FOR MAINTAINING SAID IODIDE AT A TEMPERATURE LESS THAN THE PARTS OF SAID DEVICE WITHIN SAID ENVELOPE AND BELOW 350*C., AND MEANS MAINTAINING SAID CATHODE AT A HIGHER TEMPERATURE THAN THE OTHER PARTS OF SAID DEVICE WITHIN SAID ENVELOPE SO THAT SAID IODIDE IS DISSOCIATED AT SAID CATHODE TO DISPENSE THORIUM TO SAID CATHODE DURING OPERATION OF THE DEVICE. 